The present invention relates to apparatus for increasing and decreasing the pumping capacity of a rotary screw compressor. More particularly, the present invention relates to an unloading arrangement for a refrigeration screw compressor characterized by the disposition of an unloading piston in a cylindrical bore which is remote from the compressor's working chamber but which is in flow communication with the working chamber through a series of unloader ports.
A compressor used in a refrigeration system is caused to "load" when the cooling demand placed on the evaporator of the system in which the compressor is employed increases. When the cooling demand decreases, the compressor is caused to "unload". When the load is higher, the evaporator will produce vaporized refrigerant gas at a higher rate than when the load is lower. The "load" on a compressor in a refrigeration system is therefore a function of the demand for cooling placed on the system in which the compressor is employed at any given time and is manifested in the amount of vaporized gas that the compressor is called upon to pump from the evaporator for reconditioning and re-use in the cooling process.
The screw compressor unloading arrangements of the type illustrated in U.S. Pat. Nos. 4,042,310; 4,544,333; 4,565,508; 5,203,685; and, 5,211,026, the latter two of which are assigned to the assignee of the present invention, are unloading arrangements which employ an axially moveable or rotatable unloading piston disposed within a cylindrical bore remote from the compressor's working chamber. Such unloaders are to be distinguished from "slide valve" unloaders which are more commonly used to unload screw compressors of larger capacities.
The remote bore in which piston unloaders travel communicates with the compressor's working chamber through a series of unloader ports that are aligned along the length of and open into both the unloader bore and the compressor's working chamber. Additionally, the unloader bore is in flow communication with a portion of the compressor which is at suction pressure when the compressor is in operation.
The piston unloader arrangement in co-assigned U.S. Pat. No. 5,203,685, which is incorporated herein by reference, is such that by specially configuring the geometry of a segment of the end face portion of the unloader piston and by providing apparatus by which to maintain the angular orientation of that specially configured end face portion with respect to the series of physically non-overlapping unloader ports spaced axially down the unloader bore, the unloader ports, in effect, become overlapping ports in operation while sealing within the unloader bore around the circumference of the unloader piston is accomplished. Continuous unloading of the compressor in an efficient manner is thereby achieved while internal leakage within the compressor attributable to the piston unloader arrangement is reduced.
The arrangement of the '685 patent, while an improvement over prior arrangements, requires, as noted above, the special configuration of a segment of the end face portion of the unloader piston and maintenance of a specific orientation of the unloader piston within the unloader bore in order to precisely align the specially configured end face portion with the unloader ports. The need therefore exists for a simplified and less orientation-critical arrangement for a piston unloader in a screw compressor in order to reduce parts cost, reduce alignment criticality, simplify assembly and reduce the number of potential failure modes associated with such unloader arrangements.